For increasing data transmission performance and reliability, a so called multiple-input and multiple-output technology (MIMO) may be used in wireless radio frequency telecommunications for transmitting information between a base station and a user equipment. The MIMO technology relates to the use of multiple send and receive antennas for a wireless communication at a base station and/or at a user equipment. The MIMO technology forms the basis for coding methods which use the temporal as well as the spatial dimension for transmitting information and enables therefore a space and time coding. Thus, a quality and data rate of the wireless communication may be increased.
In a so called massive MIMO system, a plurality of user equipments may be arranged within a cell served by a base station having a plurality of antennas. In the massive MIMO system, a configuration of individual antenna transceivers of the base station may vary depending on the location of each of the user equipments and transmission conditions in an environment of the base station and the user equipment.
The massive MIMO system may be used in connection with a time division duplex (TDD) system in which a transmission of an information stream between the base station and a user equipment is split up in time slots. Different time slots for uplink (UL) data communications and downlink (DL) data communications may be provided for communicating information from the user equipment up to the base station and for communicating information from the base station down to the user equipment. In the massive MIMO system, there is a need for an additional time slot which may be called “header” for transmitting a training signal or training sequence from the user equipment to the base station. Based on the received training signal, the base station may configure the transceivers of its antenna array. Thus, high antenna gain for the payload transmitted in the following time slots can be achieved. The payload may be transmitted in a number of uplink and downlink time slots. However, when the user equipment is moving, the channel quality may degrade due to a change of the spatial arrangement of the base station and/or the user equipment. Therefore, further training signals may be transmitted and the configuration of the transceivers updated.
Typically, massive MIMO systems are expected in buildings such as offices, shopping malls and so on. In this environment a large number of user equipments may be expected. However, due to the space coding by an individual configuration of the antenna parameters for each user equipment, even in this environment a high data throughput and a high data reliability may be achieved with the MIMO technology. For further enhancement, a so called 3D-MIMO system may be used in which a plurality of base stations comprising each a plurality of antennas cooperate for enabling an even more enhanced space coding of the transmitted information.
Many user equipments, for example handsets like mobile phones, provide two or even more antennas for cellular communication. The antennas of the user equipment are typically arranged spaced apart from each other at or within the housing of the user equipment, e.g. one antenna at a top and one at a bottom of the user equipment. For example, two antennas may be used in two different modes for the downlink (DL) communication, either in a diversity mode or in a MIMO mode. In the diversity mode the base station sends a single radio frequency signal and both antennas are supposed to receive it. If one of the antennas happens to be in a fading dip or if the user covers the one antenna, the other antenna still may have contact with the base station and the reception continues. The diversity mode is also called rank 1 mode. In the diversity mode the second antenna may be seen as a backup antenna. In the MIMO mode, which is also called rank 2 mode, the base station sends two different space coded signals on the very same frequency channel. Thus, the data rate may be up to two times higher. The two different radio frequency signals may be received at the two spatially separated antennas of the user equipment. A beam forming or a focusing of radio frequency signals, such that one of the two different signals may be received by a first of the two antennas of the user equipment and the other of the two different signals may be received at the same time by the other of the two antennas of the user equipment may be accomplished by the multiple antennas of the base station or the base stations operating according to the above-described massive MIMO or 3D-MIMO technology. As described above, in scenarios with rich scattered environments or a lot of user equipments, it is not possible to predict how to feed all the antennas of the base station to provide the required focusing of the radio frequency energy. Therefore, each user equipment needs to send a training sequence and then for example an amplitude and a phase for each antenna of the base station may be determined such that a radio frequency signal may be sent focused to the antenna of the user equipment from which the training sequence has been sent.
The configuration parameters for the downlink communication may be achieved based on the training sequence received in the uplink based on a channel reciprocity. With a rising transmission frequency and in rich environment scenarios with a lot of reflections and no line of sight (LOS), e.g. in indoor offices, the focusing may be very narrow, actually narrow enough to only cover one of the antennas of the user equipment. For example, the focus may be only a tenth of the wavelength of the transmission frequency. Therefore, for each antenna that needs to be covered by a beam formed by the base station, there may be a need to transmit a training sequence and therefore a transmitter for each antenna of the user equipment is needed. However, in a typical user equipment with two antennas, for cost optimization only a single transmitter may be provided, whereas two receivers are usually foreseen to enable the above-described diversity mode. Therefore, there is a need for a method enabling a transmission of data between the user equipment and the base station according to the above-described MIMO technology in a massive MIMO or 3D-MIMO system without increasing the complexity and cost of the user equipment.